1. Field of the Invention
This invention relates to heating, ventilation, air conditioning and refrigeration (“HVAC/R”) heat exchangers that reduce the resistance to airflow across coils.
2. Background Art
Many conventional heat exchangers include round tubes through which a refrigerant passes. Heat is exchanged between the refrigerant and air flowing around the outside of the tubes.
One major energy consumption consideration in HVAC/R systems is the power required to pump air through the heat exchanger. The energy required to overcome the flow resistance represents how well a heat exchanger is designed and structured. The losses in pressure are the result of the air path as it encounters tubes and airside fins. When comparing heat exchanger structures, friction factor as well as Nusselt number are usually obtained from wind tunnel tests and used to support coil design decisions. The most commonly used expression for coil pressure drop is that of Kays and London [1], which for flow normal to tube banks is:
            Δ      ⁢                          ⁢      P              P      1        =                    G        2                    2        ⁢                  g          c                      ⁢                            v          1                          P          1                    ⁡              [                                            (                              1                +                                  σ                  2                                            )                        ⁢                          (                                                                    v                    2                                                        v                    1                                                  -                1                            )                                +                                    f              ⁡                              (                                  A                                      A                    c                                                  )                                      ⁢                          (                                                v                  m                                                  v                  1                                            )                                      ]            where:                ΔP—Flow stream pressure drop        P1—Entrance pressure        G—Flow stream mass velocity        gc—Proportionality factor in Newton's second law        v1—Specific volume at entrance        v2—Specific volume at exit        
      v    m    =            (                        v          1                +                  v          2                    )        2                  σ—Ratio of free flow area to frontal area        f—Mean friction factor        A—Total heat transfer area        Ac—Minimum free flow area.Expressed alternatively:Pressure drop=flow acceleration+core friction        
The core friction portion of this relationship is made up of the entering air volume (v1), mean specific volume, the total heat transfer area (A), the free flow area (Ac) and the core friction factor. The free flow area is determined by the total tube frontal face area. By flattening the tubes and presenting the sharper tube edge to incident air and increasing Ac, the airside pressure drop can be reduced.
For reference, a commercial air handler configuration is shown in FIG. 1. Depending on customer requirements, components for filtering, heating, cooling and controlling air humidity are combined to achieve the desired room conditions. The design in FIG. 1 has a final filter (e.g., a high efficiency particulate air-HEPA filter with 99.97% efficiency), a dehumidification coil, an energy recovery wheel, UV light emitters, and five sets of modulating dampers to control the percentage of outdoor air. Motor and fan assemblies permit the system to deliver the required airflow at the specified external pressure. These components consume energy.
The consequences of combining such components in the conventional HVAC/R system are an undesirable increase in resistance to the passage of air, the consequent pressure drop and subsequent increase in energy consumption.
Further, the management and control of indoor air quality (IAQ) is a topic of high priority in the global HVAC industry. At the end of last century, several serious diseases were related to some buildings. Researchers discovered that microorganisms such as mold, bacteria, yeasts, dust mites and virus grew and spread in homes, offices, and commercial buildings through air conditioners. They observed that the recycled air inside a building may cause a Sick Building Syndrome. Uncontrolled humidity (either too high or too low) supplied a perfect environment for microorganisms.
Accordingly, in 2001, the first industrial standard, ASHRAE 62-2001, “Ventiliation for Acceptable Indoor Air Quality”, was released as a guideline for manufacturers, builders, and HVAC contractors. One consequence of meeting those standards is an increase in overall pressure drop due to additional filtration and humidification control devices.
Another factor in the HVAC industry is that the ozone-depleting refrigerant R-22, now used in most residential air conditioning systems, will be phased out by 2010. Similar programs for phasing out CFC and HCFC refrigerants in refrigeration and air conditioning systems are being implemented in Europe. Alternate refrigerants such as R-410A have been developed to replace the R-22 refrigerant. Due to higher operating pressures, R-410A systems require improved heat exchanger tubing and components.
Among the art identified in connection with a search undertaken before filing this application are the following U.S. references: U.S. Pat. Nos. 4,168,744; 4,206,806; 4,766,953; 5,123,482; 5,348,082; 5,425,414; 5,538,079; 5,604,982; 5,901,784; 6,003,592; 6,021,846; 6,044,554; 6,378,204; DE 3423746 C2; DE 3538492 A1; DE 4109127 A1; and EP 0272766 B1.